Memory controller with bank sorting and scheduling

ABSTRACT

In some embodiments a memory controller is disclosed that includes at least one command/address input buffer to receive commands and addresses. The addresses specify a memory bank and a location within the memory bank. An arbiter, coupled to the at least one command/address input buffer, merges commands and addresses from the at least one command/address input buffer and sorts the commands and addresses based on the addresses specified. A plurality of bank buffers, coupled to the arbiter and associated with memory banks, receive commands and addresses for their associated memory banks. A scheduler, coupled to the plurality of bank buffers, groups commands and addresses based on an examination of at least one command and address from the bank buffers. Other embodiments are otherwise disclosed herein.

BACKGROUND

Dynamic Random Access Memory (DRAM) is used for memory applications incomputer systems. DRAMs typically use a simple memory cell consisting ofa charge storage element (e.g., a capacitor, a floating body of atransistor) and a one or more active devices (e.g., transistors) to readfrom or write to (“access”) the charge storage element. Because thecharge storage element in each cell slowly loses charge, DRAM cells mustbe periodically refreshed.

DRAM memory cells are organized into regular arrays and are accessed(through sense amplifiers) and buffered a row (“page”) at a time and theprocess is often referred to as “opening a page”. In modern DRAMdevices, once a page is opened, one or more bits or words from theaccessed row may be read or written thereto. In many systems, a memorycontroller is used to efficiently manage the read and write transactionsbetween a processor (or processors) and one or more DRAM memory devices.

Synchronous DRAM (SDRAM) devices (e.g., double data rate (DDR)) provideincreased speed. Recent generations of DDR SDRAM (e.g., DDR2 and DDR3)have bus interface frequencies and instantaneous data rates (the columnaccess rate from an open page) ranging from 400 MHz to 800 MHz. However,the rate at which data can be written to and read from SDRAM devices isbased on a number of parameters that depend on the relatively slowprecharge and read/rewrite process required each time a row is accessed.For example, the minimum time period from the start of a row access tothe start of a new row access (the row-cycle time (tRC)) may range fromabout 45 nS to about 60 nS (data rate in the range of about 16-22 MHz).

DDR SDRAM devices may use multiple memory cell arrays (“banks”), witheach bank having its own sense amplifiers and buffering logic toincrease performance. Some current DDR SDRAM devices support as many as8 banks per device. Multi-bank SDRAM devices allow for the access of anew row of memory data from one bank while reading the data from an openpage of another bank. Once a row within a particular bank is activated(opened), it is most efficient to get as many consecutive accesses todifferent columns within that same row. However, access to a differentrow within that bank may be limited by the tRC or other row accessparameters.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the various embodiments will becomeapparent from the following detailed description in which:

FIG. 1 illustrates an example simplified system block diagram of acomputer, according to one embodiment;

FIGS. 2A-C illustrate example memory controllers, according to oneembodiment;

FIG. 3 illustrates an example round robin scheduling process, accordingto one embodiment;

FIG. 4 illustrates an example weighted round robin scheduling process,according to one embodiment; and

FIG. 5 illustrates an example priority scheduling process, according toone embodiment.

DETAILED DESCRIPTION

FIG. 1 is a simplified functional block diagram of an examplemicroprocessor-based computer system 100. The computer system 100includes a processor (central processing unit (CPU)) 110, a memorycontroller 120, system memory 130, an input/output (I/O) controller 140,I/O ports 150, and Peripheral Component Interconnect bus (PCI) slots 160adhering to the PCI Local Bus Specification Revision 2.1 developed bythe PCI Special Interest Group of Portland, Oreg. Other componentstypically used in the computer system 100, but not illustrated, includeone or more hard disk drives, one or more optical disk drives (e.g.,CD-ROM, DVD-ROM), one or more network interfaces, a video/graphicsinterface and adapter, a video monitor, and a keyboard. A power supply(not shown) is also required to provide one or more DC voltagesappropriate for use by the various components of the computer system100.

The processor 110 may be a traditional processor. For example, theprocessor 110 may be a particular member of the Intel® family ofprocessors, including the Pentium® II, Pentium® III, Pentium® IV,Pentium® 4 Processor-M, and Itanium processors available from IntelCorporation of Santa Clara, Calif. The processor 110 may be a networkprocessor. The processor 110 may be a single processor or may bemultiple processors. If the processor 110 is multiple processors, themultiple processors may consist of multiple chips, may consist of asingle chip with multiple processors (multi-core processor), or somecombination thereof. If multiple processors the processors may be thesame type or may be of a different type.

The system memory 130 stores data and program instructions that may beused by the processor 110. The system memory 130 may include dynamicrandom access memory (DRAM) or may be implemented using other memorytechnologies. The I/O controller 140, coupled to the memory controller120, provides an interface to most I/O devices within the computersystem 100. The I/O controller 140 may be coupled to one or more of theI/O ports 150, which may include RS-232 serial ports, parallel ports,and Universal Serial Bus (USB) ports. The USB ports are specified by theUniversal Serial Bus Revision 1.1 specification or the Universal SerialBus Revision 2.0 specification, both from the USB Implementers Forum,Inc. of Portland, Oreg. The I/O controller 140 may also be coupled toone or more of the PCI slots 160.

The functional blocks of FIG. 1 are not intended to illustrate aparticular partitioning of functionality into integrated circuits (ICs).Rather, the functionality may be partitioned into ICs in any mannerwithout departing from the scope. For example, the memory controller 120may be a separate IC or may be incorporated on the same die as theprocessor 110. The memory controller 120 may be incorporated onto one ormore ICs that form the system memory 130.

FIG. 2A illustrates an example memory controller 200 (e.g., 120 of FIG.1). The memory controller 200 may support coherent streams havingsequential or nearly sequential memory access (e.g., streams from atraditional processor), non-coherent streams having non-sequential orrandom memory access (e.g., streams from a network processor), and/ordifferent types of streams with various memory access characteristicsand requirements.

The memory controller 200 includes a command/address First-In-First-Outbuffer (FIFO) 210, an arbiter 220, a bank FIFO set 230 having pluralityof bank FIFOs (labeled 0 to N−1), a bank scheduler 240, a pin statemachine 250, an internal command generator 255, a data path and steeringlogic 260, an ECC logic 265, a write buffer 270, and a read FIFO 280.The memory controller 200 may service commands (requests to read orwrite data) from one or more masters (e.g., processor 110 of FIG. 1).The commands and addresses associated therewith enter the memorycontroller 200 and are buffered in the command/address FIFO 210. Readrequests may be tagged to allow proper association of read requests anddata from one or more memory devices (not shown).

The outputs from command/address FIFO 210 are fed into the arbiter 220.The arbiter 220 sorts memory requests into appropriate bank FIFOs fromthe bank FIFO set 230. The arbiter 220 may use a simple round robinarbitration scheme to sort and prioritize the input request streams. Thearbiter 220 may also arbitrate between the memory requests and commandsfrom an internal command generator 255 (discussed later). Theappropriate bank may be determined by examination of one or more addressbits in each command/address input. The sorted requests are fed into theappropriate bank FIFOs from the bank FIFO set 230. The number of bankFIFOs is equal to the number (N) of banks in the target memory devices(not shown). For example, where the target memory devices contain eightbanks there are eight bank FIFOs.

The bank scheduler 240 receives the outputs from the bank FIFO set 230.The bank scheduler 240 processes the requests in rounds. In each round,the bank scheduler 240 may select the transactions that optimizeread/write efficiency and maximize the use of memory “pin” bandwidth.The bank scheduler 240 may minimize bank conflicts by sorting,reordering, and clustering memory requests to avoid back-to-backrequests of different rows in the same bank. The bank scheduler 240 mayavoid requests of different rows in the same bank for at least therow-cycle time (tRC), which is the minimum time period required betweenthe start of a row access to the start of a new row access, so that thetRC does not effect the speed by which the requests are processed.

The bank scheduler 240 may also group reads and/or writes to minimizeread-write turn-arounds. For example, up to eight like transactions maybe collected before switching to the other type (e.g., from read towrite, from write to read). The bank scheduler 240 may select either allreads or all writes targeted to different banks and schedule thesetransactions for a particular round of scheduling.

The bank scheduler 240 may also maintain a tRC timer for each bank. ThetRC timer for a specific bank may be started when a request is issued tothe specific bank. The bank scheduler 240 will not allow another requestfor the specific bank for at least a time equal to the tRC. The bankscheduler 240 may set the tRC timer to the tRC time and have it countdown to zero, or may reset the tRC time to zero and have it count up totRC. The specific bank becomes eligible again to receive a newtransaction after the time equal to the tRC (e.g., timer reaches 0,timer reaches tRC).

For each round the bank scheduler 240 may select a specific transactiontype (e.g., read, write) from each bank FIFO in the bank FIFO set 230that have an associated tRC value indicating the associated bank FIFO iscapable of performing a next transaction (e.g., zero, tRC) and havingthe specific transaction type (e.g., read, write) at the head of theassociated bank FIFO. The bank scheduler 240 may select up to a certain(e.g., 8) number of the specific transaction types. The bank scheduler240 may be configured to switch the transaction type at the beginning ofeach new round. For each round, the bank scheduler 240 may maintain acount of the number of bank FIFOs skipped because the transaction at thehead of the FIFO is not of the correct type (e.g., read instead ofwrite, write instead of read). The bank scheduler 240 may be programmedto switch if the skip count is greater than a certain value.

The bank scheduler 240 may examine transactions further into each FIFObank and consider more than just the head element as a candidate to bescheduled (“look-at-N scheduler”, where N can be any integer from 2 tothe size of the bank FIFO). The look-at-N bank scheduler 240 may scanthe first N elements of each bank FIFO to pick a specific transactiontype. For example, if the specific transaction type for a given round isa “read” and N=3, the look-at-N bank scheduler 240 may select a readtransaction from a first (head), second or third transaction in the bankFIFOs having an appropriate tRC timer value (e.g., 0, tRC). Thelook-at-N bank scheduler 240 increases the probability of finding therequired transaction-type since multiple elements from each bank FIFOare scanned.

The look-at-N bank scheduler 240 enables read transactions to bypasswrite transactions or writes to bypass reads. An “out-of-order”mechanism may be used to ensure that the transaction ordering rulesgoverning reads and writes to the same address are never violated. Theout-of-order mechanism may also ensures that reads are not allowed tobypass other reads and writes are not allowed to bypass other writeswithin the bank FIFO. The out-of-order mechanism may tag each incomingread request (for coherent streams) and provide a score-board mechanismto buffer read data returned from the memory devices (not shown). Thetag for each unit of returned data may be compared with the tags storedin the scoreboard, and the data may be sorted in age order. Thescoreboard ensures that the data for the oldest read request is alwaysreturned ahead of data for newer read requests.

The output of bank scheduler 240 is processed by the pin state machine250 to produce address, command, and control signals necessary to sendread and write transactions to the attached memory devices (not shown).The internal command generator 255 performs maintenance functions,including DRAM refresh generation, correcting single bit errorcorrection (ECC) errors encountered upon DRAM reads, and periodic memoryscrubbing to find ECC errors that may have developed in DDR locationsnot recently read. Since the maintenance functions require little memorybandwidth, they arbitrate for access to the bank fifos (via the arbiter220) in a round robin fashion with the primary request streams receivedby the command/address FIFO 210.

Write data enters the memory controller 200 through the write buffer270. The write data may be merged into the data path and steering logic260, processed by the ECC logic 265, and forwarded via a data bus todata pins of the memory devices (not shown). Data being read from thememory devices is received from the data bus and processed by the ECClogic 250. The read data is distributed, and possibly reordered, by thedata path and steering logic 260 to the appropriate processors. The datapath and steering logic 260 receives the read data in the order which itwas accessed from the memory devices. The order may not be the same asthe order in which the read commands were presented from a processorbecause the scheduler may issue commands to the DRAM in an out of ordersequence, in order to maximize DRAM bandwidth.

The data path and steering logic 260 determines where the read data isbeing routed to (e.g., what type of processor requested the data). A noncoherent processor (e.g., network processor) can receive the reads outof order and reorder the data, based on sequence tags applied to boththe original read command, and the associated read data. Accordingly,the data path and steering logic 260 simply routes the read data to thenon-coherent processor that initiated the read transaction via the readFIFO 280. A coherent processor (e.g., traditional processor) mustreceive the reads in the same order the read commands were presented.Accordingly, the data path and steering logic 260 enqueues the read dataand performs the reordering, based on sequence tags associated with boththe original read requests, and the associated read return data androutes the reordered data to coherent processor that initiated the readtransaction via the read FIFO 280.

FIG. 2B illustrates an example memory controller 202 that may servicecommands (requests to read or write data) from two or more masters(processors). The masters may be coherent processors, non-coherentprocessors, other types of processors, or some combination thereof. Forease of description and for clarity we will discuss the memorycontroller 202 receiving commands from two processors, a “coherent”processor A and a “non-coherent” processor B). However, the variousembodiments are not limited thereto.

The memory controller 202 includes command/address FIFO A 212,command/address FIFO B 214, an arbiter 222, a write buffer A 272, awrite buffer B 274, a read FIFO A 282, a read FIFO B 284, as well as thebank FIFO set 230 having plurality of bank FIFOs (labeled 0 to N−1), thebank scheduler 240, the pin state machine 250, the internal commandgenerator 255, the data path and steering logic 260, and the ECC logic265.

The coherent stream commands and addresses enter the memory controller202 via a coherent input and are buffered in the command/address FIFO A212. The non-coherent stream commands and addresses enter memorycontroller 202 via a non-coherent input and are buffered incommand/address FIFO B 214. Both the coherent and non-coherent readrequests may be tagged to allow proper association of read requests anddata from one or more memory devices (not shown).

The outputs from command/address FIFO A 212 and command/address FIFO B214 are fed into the arbiter 222. The arbiter 222 may use a simple roundrobin arbitration scheme to merge the coherent and non-coherent inputrequest streams. In other embodiments, a more complex arbitrationscheme, such as weighted round robin, may be used. The arbiter 222 mayalso receive commands from the internal command generator 255 andarbitrate between the commands and the requests.

The coherent write data enters the memory controller 202 through thewrite buffer A 272 and the non-coherent write data enters the memorycontroller 202 through the write buffer B 274. The data path andsteering logic 260 enqueues the coherent read data and performs thereordering, based on sequence tags associated with both the originalread requests, and the associated read return data and routes thereordered data to the coherent processor that initiated the readtransaction via the read FIFO A 282. The data path and steering logic260 simply routes the non-coherent read data to the non-coherentprocessor via the read FIFO B 284.

The bank FIFO set 230, the bank scheduler 240, the pin state machine250, the internal command generator 255, the data path and steeringlogic 260, and the ECC logic 265 perform the same or similar functionsto those described with respect to FIG. 2A

FIG. 2C illustrates an example memory controller 204 that may servicecommands from two or more masters. Like FIG. 2B for ease of descriptionand for clarity we will discuss the memory controller 204 receivingcommands from two processors, a “coherent” processor A and a“non-coherent” processor B. The memory controller 204 includes anarbiter A 224, an arbiter B 226, a bank FIFO set A 232, a bank FIFO setB 234, a bank scheduler 245 as well as the command/address FIFO A 212,the command/address FIFO B 214, the pin state machine 250, the internalcommand generator 255, the data path and steering logic 260, the ECClogic 265, the write buffer A 272, the write buffer B 274, the read FIFOA 282, and the read FIFO B 284.

The pin state machine 250, the internal command generator 255, the datapath and steering logic 260, and the ECC logic 265 perform the same orsimilar functions to those described with respect to FIGS. 2A and 2B.The command/address FIFO A 212, the command/address FIFO B 214, thewrite buffer A 272, the write buffer B 274, the read FIFO A 282, and theread FIFO B 284 perform the same or similar functions to those describedwith respect to FIG. 2B.

The output of FIFO A 212 is fed into the arbiter A 224 and the output ofFIFO B 214 is fed into the arbiter B 226. The arbiter A 224 and thearbiter B 226 may use a simple round robin arbitration scheme to sortand prioritize the input coherent and non-coherent request streamsrespectively. The arbiters A and B 224, 226 may also arbitrate betweenthe memory requests and commands from the internal command generator255. The arbiters A and B 224, 226 sort the memory requests intoindividual banks, where the banks may be determined by examination ofone or more address bits in each command/address input. The coherentrequests sorted by arbiter A 224 are fed into an appropriate bank FIFOin the bank FIFO set A 232 and the non-coherent requests sorted byarbiter B 226 are fed into the appropriate bank FIFO in the bank FIFOset B 234. The bank FIFO set A 232 and the bank FIFO set B 234 eachcontain “N” FIFOs, where “N” is the number of banks in the target memorydevice (not shown).

The bank scheduler 245 receives the outputs from the bank FIFOs in thebank FIFO set A 232 and the bank FIFO set B 234. The bank scheduler 245,like the bank scheduler 240 of FIGS. 2A and 2B, picks the transactionsthat optimize read/write efficiency and maximize the use of memory “pin”bandwidth. The bank scheduler 245 may minimize bank conflicts bysorting, reordering, and clustering memory requests to avoidback-to-back requests of different rows in the same bank within the tRCwindow. The bank scheduler 245 may also group reads and/or writes tominimize read-write turn-arounds. In each round, the bank scheduler 245may select either all reads or all writes targeted to different banksand schedule these transactions. The bank scheduler 245 may alsomaintain a tRC timer for each bank. A bank tRC timer is started when arequest is issued to that bank and the bank becomes eligible again toreceive a new transaction when the timer counts down to zero.

The bank scheduler 245 may be configured to switch the transaction typeat the beginning of each new round. For each round, the bank scheduler245 may maintain a count of the number of bank FIFOs skipped because thetransaction at the head of the FIFO is not of the correct type (e.g.,read instead of write, write instead of read). The bank scheduler 245may be programmed to switch if the skip count is greater than a certainvalue. The bank scheduler 245 may examine transactions further into eachFIFO and consider more than just the head element as a candidate for theschedule (“look-at-N scheduler”).

The bank scheduler 245 may arbitrate between coherent transactionrequests from the bank FIFO set A 232 and non-coherent transactionrequests from the bank FIFO set B 234. Arbitrating between coherent andnon-coherent transaction requests may provide improved performance wherethere is a mismatch in the arrival rate of requests to the memorycontroller 204. In particular, this may overcome unfair bandwidthallocation problems when there is a significant mismatch in the arrivalrate.

In each round, the bank scheduler 245 may schedule either readtransactions or write transaction. In a read round, each bank FIFOwithin the bank FIFO set A 232 and the bank FIFO set B 234 produces acandidate request if the transaction at the head of the FIFO is a readtransaction. Once all the read candidates are determined, the bankscheduler 245 makes scheduling decisions based on a number of criteria.A history bit may be used for each bank to store which bank FIFO(coherent FIFO or non-coherent FIFO) was selected in the last scheduledround. Scheduling may be performed using a simple round robin scheme.

FIG. 3 illustrates an example round robin read transactiondecision-making process. The scheduling decision process begins bychecking if the bank's tRC timer has elapsed (300). If the timer has notelapsed (300 No), the selected bank is not ready to accept a newtransaction so no transactions for the current bank are processed andthe process advances to the next bank (310). If the timer has elapsed(300 Yes), then a determination is made as to whether the bank is withina rolling time window (tFAW) limit—no more than four banks have beenactivated within tFAW (320). If the bank tFAW limit has been exhausted(320 No), then no transactions for the current bank are processed andthe process advances to the next bank (310). If the bank is still withinthe tFAW limit (320 Yes), then the process determines if there are bothcoherent and non-coherent requests pending (330).

If only one type of request is pending (330 No), then that transactionis scheduled (340). If both types of requests are pending (330 Yes),then a determination is made as to whether the type of request for thelast round was coherent (350). If the transaction in the last round wasnot a coherent one (350 No), then a coherent transaction is scheduled(360). If the transaction in the last round was a coherent one (350Yes), then a non-coherent transaction is scheduled (370). Afterscheduling a transaction, the process advances to the next bank (310).

Rather than using a simple round robin process, the scheduling may beperformed using a weighted round robin process. A history state (statecount), comprising an M-bit number, may be kept for each bank toidentify a sequence of 2^(M) rounds. Some fraction of the 2^(M) roundsmay be set aside for coherent transactions and the remaining rounds areused for non-coherent transactions. By appropriately selecting therelative weights (proportion of rounds), the available memory bandwidthcan be fairly distributed between (or among) slower and fastertransaction request streams. For example, a 2-bit history state wouldenable the WRR to assign different weights to coherent and non-coherentrequests for a set of four (2²) rounds (e.g., 1 round for non-coherenttransactions and three for coherent, 1 round for coherent transactionsand three for non-coherent). Larger values of M allow for a finergrained weighting.

FIG. 4 illustrates an example weighted round robin read transactiondecision-making process. The scheduling decision process begins bychecking if the bank's tRC timer has elapsed (400). If the timer has notelapsed (400 No), the process advances to the next bank (410). If thetimer has elapsed (400 Yes), then the tFAW parameter is checked todetermine if it is within the limit (420). If the bank tFAW limit hasbeen exhausted (420 No), then the process advances to the next bank(410). If the tFAW parameter is within limit (420 Yes), then the processdetermines if there are both coherent and non-coherent requests pending(430). If only one type of request is pending (430 No), then thattransaction is scheduled (440).

If both types of requests are pending (430 Yes), then the state count ischecked to determine if it indicates a coherent transaction (450). Ifthe state count indicates a non-coherent transaction (450 No), then anon-coherent transaction is scheduled (460). If the state countindicates a coherent transaction (450 Yes), then a coherent transactionis scheduled (470). The state count is then incremented (480) and theprocess advances to the next bank (410).

The bank scheduler 245 may provide higher priority to a slower stream(e.g., the coherent stream is slower than the non-coherent stream). Thisensures that, in any round, the slower stream is always selected beforethe faster stream. This scheduling scheme provides the best performancefor the slower transaction stream.

FIG. 5 illustrates an example priority based transaction decision-makingprocess. The scheduling decision process begins by checking if thebank's tRC timer has elapsed (400). If the timer has not elapsed (500No), the process advances to the next bank (510). If the timer haselapsed (500 Yes), then a determination is made as to whether the bankis within the tFAW limit (520). If out of limit (520 No), then theprocess advances to the next bank (510). If within limit (520 Yes), thenthe process determines if there are coherent requests pending (530). Ifno coherent requests are pending (530 No), then a non-coherenttransaction is scheduled (540). If a coherent request is pending (530Yes), then that transaction is scheduled (550). The process thenadvances to the next bank (510).

Although the various embodiments have been illustrated by reference tospecific embodiments, it will be apparent that various changes andmodifications may be made. Reference to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment” appearing in various places throughoutthe specification are not necessarily all referring to the sameembodiment.

Different implementations may feature different combinations ofhardware, firmware, and/or software. It may be possible to implement,for example, some or all components of various embodiments in softwareand/or firmware as well as hardware, as known in the art. Embodimentsmay be implemented in numerous types of hardware, software and firmwareknown in the art, for example, integrated circuits, including ASICs andother types known in the art, printed circuit broads, components, etc.

The various embodiments are intended to be protected broadly within thespirit and scope of the appended claims.

1. A memory controller comprising: at least one coherent command/addressinput buffer to receive commands and addresses for coherent streamshaving sequential or nearly sequential memory access, wherein theaddresses specify a memory bank and a location within the memory bank;at least one non-coherent command/address input buffer to receivecommands and addresses for non-coherent streams having non-sequential orrandom memory access; at least one coherent arbiter, coupled to said atleast one coherent command/address input buffer, to merge the commandsand addresses from said at least one coherent command/address inputbuffer and sort the commands and addresses based on the addressesspecified; at least one non-coherent arbiter, coupled to said at leastone non-coherent command/address input buffer, to merge the commands andaddresses from said at least one non-coherent command/address inputbuffer and sort the commands and addresses based on the addressesspecified; at least one coherent plurality of bank buffers, coupled tothe at least one coherent arbiter and associated with memory banks, tostore the commands and addresses for the associated memory banks; atleast one non-coherent plurality of bank buffers, coupled to the atleast one non-coherent arbiter, to store the commands and addresses forthe associated memory banks; and a scheduler, coupled to said at leastone coherent plurality of bank buffers and at least one non-coherentplurality of bank buffers, to select the commands and addresses fromsaid at least one coherent plurality of bank buffers and at least onenon-coherent plurality of bank buffers to transact, wherein saidscheduler is to operate in rounds and is to select a specific commandtype per round, wherein during each round said scheduler is to onlyselect the commands and addresses having the specific command type fromsaid at least one coherent plurality of bank buffers and at least onenon-coherent plurality of bank buffers, and wherein said scheduler is toarbitrate between said at least one coherent plurality of bank buffersand said at least one non-coherent plurality of bank buffers.
 2. Thememory controller of claim 1, wherein said scheduler is to examine afirst transaction in each one of said at least one coherent plurality ofbank buffers and at least one non-coherent plurality of bank buffersconsidered during the round to determine if the first transaction is thespecific command type.
 3. The memory controller of claim 1, wherein saidscheduler is to examine N transactions, where N>1, in each one of saidat least one coherent plurality of bank buffers and at least onenon-coherent plurality of bank buffers considered during the round todetermine if any of the N transactions are the specific command type. 4.The memory controller of claim 1, wherein said scheduler includes aplurality of tRC timers associated with respective ones of said at leastone coherent plurality of bank buffers and at least one non-coherentplurality of bank buffers, wherein said scheduler is to initiate a tRCtimer after a respective bank buffer performs a transaction, and whereinsaid scheduler restricts selection of a transaction from the respectivebank buffer for a period of at least tRC.
 5. The memory controller ofclaim 1, wherein said scheduler is to change the command type per round.6. The memory controller of claim 1, wherein said at least one coherentcommand/address input buffer associates a tag to received commands andaddresses.
 7. The memory controller of claim 1, wherein said scheduleruses a simple round robin arbitration.
 8. The memory controller of claim1, wherein said scheduler uses a weighted round robin arbitration. 9.The memory controller of claim 1, wherein said scheduler uses simplepriority arbitration.
 10. A memory controller comprising: at least onecommand/address input buffer to receive commands and addresses, whereinthe addresses specify a memory bank and a location within the memorybank; an arbiter, coupled to said at least one command/address inputbuffer, to merge the commands and addresses from said at least onecommand/address input buffer and sort the commands and addresses basedon the addresses specified; a plurality of bank buffers, coupled to thearbiter and associated with memory banks, to store the commands andaddresses for their associated memory banks; and a scheduler, coupled tosaid plurality of bank buffers, to select the commands and addressesfrom said plurality of bank buffers to transact, wherein said scheduleris to operate in rounds and is to select a specific command type perround, wherein during each round said scheduler is to only select thecommands and addresses having the specific command type from saidplurality of bank buffers, and wherein said scheduler is to track anumber of said bank buffers that were not processed per round becausethey had a wrong command type and is to end the round and change thecommand type when the number reaches a certain level.
 11. A methodcomprising: receiving coherent commands and addresses from at least onecoherent source having sequential or nearly sequential memory access,wherein the addresses specify a memory bank and a location within thememory bank; receiving non-coherent commands and addresses from at leastone non-coherent source having non-sequential or random memory access;merging the received coherent commands and addresses; merging thereceived non-coherent commands and addresses; sorting the mergedcoherent commands and addresses by memory bank; sorting the mergednon-coherent commands and addresses by memory bank; buffering thecoherent commands and addresses in coherent bank buffers associated withmemory banks; buffering the non-coherent commands and addresses innon-coherent bank buffers associated with the memory banks; andscheduling the coherent commands and addresses and the non-coherentcommands and addresses to be transacted, wherein said scheduling isperformed in rounds by specific command type, wherein said schedulingfor each round is limited to the coherent commands and addresses withinthe coherent bank buffers and the non-coherent commands and addresseswithin the non-coherent bank buffers having the specific command type,and wherein said scheduling further includes arbitrating between thecoherent commands and addresses and the non-coherent commands andaddresses.
 12. The method of claim 11, wherein said scheduling includesexamining first N transactions, where N>1, in each of the coherent bankbuffers and non-coherent bank buffers considered during the round todetermine if any of the first N transactions are the specific commandtype.
 13. A memory controller comprising: at least one coherentcommand/address input buffer to receive commands and addresses forcoherent streams having sequential or nearly sequential memory accessand at least one non-coherent command/address input buffer to receivecommands and addresses for non-coherent streams having non-sequential orrandom memory access, wherein the addresses specify a memory bank and alocation within the memory bank; at least one coherent arbiter,associated with and coupled to said at least one coherentcommand/address input buffer, and at least one non-coherent arbiterassociated with and coupled to said at least one non-coherentcommand/address input buffer, to sort the commands and addresses basedon the addresses specified; at least one coherent set of bank buffers,associated with and coupled to said at least one coherent arbiter, andat least one non-coherent set of bank buffers, associated with andcoupled to said at least one non-coherent arbiter, to buffer thecommands and addresses for their associated memory banks; and ascheduler, coupled to the at least one coherent set of bank buffers andthe at least one non-coherent set of bank buffers, to select thecommands and addresses from the at least one coherent set of bankbuffers and the at least one non-coherent set of bank buffers totransact, wherein said scheduler is to operate in rounds and is toselect a specific command type per round, and wherein during each roundsaid scheduler is to only select the commands and addresses having thespecific command type from the at least one coherent set of bank buffersand the at least one non-coherent set of bank buffers and is toarbitrate between the at least one coherent set of bank buffers and theat least one non-coherent set of bank buffers.
 14. The memory controllerof claim 13, wherein said scheduler is to examine N transactions, whereN>1, in each one of said at least one coherent set of bank buffers andsaid at least one non-coherent set of bank buffers considered during theround to determine if any of the N transactions are the specific commandtype.
 15. A computer comprising: a Dynamic Random Access Memory (DRAM)device; a processor; and a memory controller comprising: at least onecoherent command/address input buffer to receive commands and addressesfor coherent streams having sequential or nearly sequential memoryaccess and at least one non-coherent command/address input buffer toreceive commands and addresses for non-coherent streams havingnon-sequential or random memory access, wherein the addresses specify amemory bank within said DRAM device and a location within the memorybank; at least one coherent arbiter, coupled to said at least onecoherent command/address input buffer, to merge the commands andaddresses from said at least one coherent command/address input bufferand sort the commands and addresses based on the addresses specified; atleast one non-coherent arbiter, coupled to said at least onenon-coherent command/address input buffer, to merge the commands andaddresses from said at least one non-coherent command/address inputbuffer and sort the commands and addresses based on the addressesspecified; at least one coherent plurality of bank buffers, coupled tothe at least one coherent arbiter and associated with memory banks, tobuffer the commands and addresses for the associated memory banks; atleast one non-coherent plurality of bank buffers, coupled to the atleast one non-coherent arbiter, to buffer the commands and addresses forthe associated memory banks; and a scheduler, coupled to said at leastone coherent plurality of bank buffers and at least one non-coherentplurality of bank buffers, to select the commands and addresses fromsaid at least one coherent plurality of bank buffers and at least onenon-coherent plurality of bank buffers to transact, wherein saidscheduler is to operate in rounds and is to select a specific commandtype per round, wherein during each round said scheduler is to onlyselect the commands and addresses having the specific command type, andwherein said scheduler is to arbitrate between said at least onecoherent plurality of bank buffers and said at least one non-coherentplurality of bank buffers.